This invention relates to an electronic fuel injection system for an internal combustion engine and more particularly to an electronic fuel injection system which has a vacuum sensor for sensing the intake manifold pressure, in addition to or without a throttle position sensor, so as to actuate fuel enrichment properly especially for the engine operating ranges associated with low and high speed loads.
In a conventional electronic fuel injection system, a throttle valve switch produces a fuel enrichment correction signal in response to the throttle valve exceeding a predetermined opening degree. An injector open-valve time period is increased in response to this correction signal to achieve the fuel enrichment. However, usually the throttle valve is not opened to such an extent as to actuate a throttle valve switch for a relatively low speed operational range, as in the case of running in an urban district.
If the amount of exhaust gas recirculation (EGR) is increased to reduce exhaust gas nitrogen oxides (NOx) for relatively low speeds and relatively high loads, various undesirable results occur because the EGR amount is increased without fuel enrichment. Examples of the undesirable results are: deterioration in performance, troubles resulting from intake passage overheating and formation of hard deposits, such as carbon sediments which cause part sticking and clogging of the intake passage. On the other hand, if the air/fuel ratio is always maintained at a richer level to avoid such problems, then fuel comsumption is badly influenced.